Problem: $405$ is what percent of $900$ ?
Explanation: Having a percent of something means that you get that percent out of every $100$ We can set up a proportion to find out what percent of $900$ we need to take to get $405$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{\text{part}}}{{\text{whole}}}$ Which things do we know, and what are we trying to find? We are trying to find the ${\text{percent}}$ . Is $900$ the ${\text{part}}$ or the ${\text{whole}}$ The $900$ is the ${\text{whole}}$ . This means the ${\text{part}}$ is $405$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{405}}{{900}}$ If we divide the denominator of the fraction on the right by $9$ , it will be the same denominator of the fraction on the left. To keep things equal, let's also divide the numerator on the right by $9$ $ \dfrac{{\text{percent}}}{100 } = \dfrac{{405 \div 9}}{{900 \div 9}}$ $ \dfrac{{\text{percent}}}{100 } = \dfrac{{45}}{{100}}$ $ {\text{percent}} = {45}$ So $405$ is $45\%$ of $900$.